1. Field of the Invention
The present invention relates to a communication system, a communication apparatus, a communication method, and a program executing communication processing that is advantageously applicable to a wireless ad hoc network in which a wireless LAN (Local Area Network) system for use in a data communication performs operation without a master control station.
2. Description of Related Art
One of widely known conventional media access control methods used in a wireless LAN system is an access control method defined by the IEEE (The Institute of Electrical and Electronics Engineers) 802.11 standard. The IEEE 802.11 standard is described in detail in International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, and so on.
Networking conforming to the IEEE 802.11 standard is based on the concept of BSS (Basic Service Set). There are two types of BSS, wherein one is BSS defined in an infrastructure mode in which the system has a master control station such as an access point (AP) and the other is IBSS (Independent BSS) defined in an ad hoc mode in which the system is composed of multiple mobile terminals only (Mobile Terminal: MT). For communication in the ad hoc mode, it is known that communication is performed in the CSMA (Carrier Sense Multiple Access) mode using the RTS/CTS procedure. This processing is employed also by the IEEE 802.11 standard described above.
The following describes an example of the RTS/CTS procedure operation with reference to FIGS. 12A and 12B. FIGS. 12A and 12B are diagrams showing the conventional access control procedure executed by exchanging the RTS/CTS signals. As shown in FIG. 12A, the transmission side communication apparatus transmits the RTS signal before transmitting data and, upon receiving the CTS signal from the reception side communication apparatus shown in FIG. 12B, can start transmitting data.
In addition, the ACK signal, an acknowledgement signal indicating that data is received correctly, is usually returned from the reception side communication apparatus immediately after the data transmission is terminated, as shown in FIG. 12B. By transmitting and receiving the RTS signal and the CTS signal before starting data transmission in this way, communication can be performed while confirming if data can be transmitted through the wireless transmission line.
The Patent publication 1 discloses an example in which the RTS signal and the CTS signal are transmitted and received for performing wireless communication access control.
[Patent Publication 1]
Japanese Laid-Open Patent JPH8-37528.
However, even when the RTS signal and the CTS signal are transmitted and received before the data is transmitted through an ad hoc network, the data transmission sometimes fails if there is a so-called hidden terminal apparatus. FIGS. 13A to 13C show an example of a collision that is caused when the conventional RTS/CTS signal exchange is used to perform the access control. When the RTS signal is transmitted from the transmission side communication apparatus, the reception side communication apparatus returns the CTS signal in response to the RTS signal and, immediately after that, receives data as shown in FIGS. 13A and 13B.
Now, assume that there is a so-called hidden terminal apparatus that can not directly communicate with the transmission side communication apparatus through a wireless communication line, but can directly communicate with the reception side communication apparatus, as shown in FIG. 13C. At this time, if the hidden terminal apparatus regularly transmits a beacon signal to notify its presence to some other communication apparatus in the network, there is a possibility that the transmission timing of the beacon signal coincides with the transmission timing of data transmission from the transmission side communication apparatus as shown in FIGS. 13A to 13C. The hidden terminal apparatus is able to temporarily stop the transmission of the beacon signal if the hidden terminal apparatus can receive the RTS signal from the transmission side communication apparatus. However, because the hidden terminal apparatus in this example cannot receive the RTS signal from the transmission side communication apparatus, there exits a problem that cannot be solved.
One of the solutions of this problem is that, if there is a possibility of a collision between the reception of data and the reception of the beacon signal, it is thought that the reception side communication apparatus does not return the CTS signal to prevent the data transmission from being started. That is, the transmission side communication apparatus transmits the RTS signal prior to the transmission of data, for example, as shown in FIG. 14A. In this case, the reception side communication apparatus that receives the RTS signal does not return the CTS signal but enters the waiting state as shown in FIG. 14B (The broken line indicates the timing at which the CTS signal is to be transmitted). Although this method prevents the transmission side communication apparatus from transmitting data, the transmission side communication apparatus transmits the RTS signal again because it has a request to transmit data. However, there is a higher possibility of a collision between the transmission of the RTS signal and the transmission of the beacon signal transmitted from the hidden terminal apparatus shown in FIG. 14C. This collision between the RTS signal and the beacon signal prevents the reception side communication apparatus from correctly identifying the RTS signal, further delaying the chance for the data communication.
To solve this problem, the patent publication 1 described above discloses a technology that, if there is a possibility that the reception side communication apparatus receives the RTS signal but cannot receive data that will be transmitted following the RTS signal, the reception side communication apparatus returns the cancel signal instead of the CTS signal to cancel the data transmission at that time. The cancel signal disclosed in the patent publication 1 may also be used to return the cancel signal to cancel the RTS signal, for example, when the time at which the hidden terminal apparatus transmit the beacon signal is known.
However, because the transmission side communication apparatus that receives the cancel signal has data to be transmitted to the opponent, the transmission side communication apparatus must transmit the RTS signal again after a predetermined time to retry the communication. FIGS. 15A to 15C are diagrams showing an example of the above case. For example, the transmission side communication apparatus transmits the RTS signal before the data transmission as shown in FIG. 15A. In this case, the reception side communication apparatus returns the cancel signal in response to the RTS signal as shown in FIG. 15B. Then, after a predetermine time, the transmission side communication apparatus transmits the RTS signal again. However, if the hidden terminal apparatus starts the transmission of the RTS signal or data to some other terminal as shown in FIG. 15C, the retransmission of the RTS signal from the transmission side communication apparatus may collide with the transmission of the RTS signal or data from the hidden terminal apparatus.
As described above, the collision with the transmission from a hidden terminal apparatus is not avoided sufficiently and simply by delaying the transmission. Therefore, it is hard to say that the collision avoiding processing is carried out very efficiently.